Polyimides of benzophenone tetracarboxylic acid compound and 3,9-bis(3-aminopropyl)-2,4,8,10-tetraoxaspiro(5,5)-undecane and method of preparation

ABSTRACT

A POLYAMIDE HAVING A RECURRING STRUCTURAL UNIT OF THE FORMULA   (-CH2-O-CH(-(CH2)2-CH2(-))-O-CH2-)&gt;C&lt;(-CH2-O-CH(-(CH2)3-   (1,3-DI(O=)ISOINDOLIN-2,5-YLENE)-CO-(1,3-DI(O=)ISOINDOLIN-   5,2-YLENE)(-))-O-CH2-)   CAN BE PRODUCED BY CAUSING A 3,3&#39;&#39;,4,4&#39;&#39;-BENZOPHENONETETRACARBOXYLIC ACID COMPOUND AND 3,9-BIS(3-AMINOPROPYL)-2,4,8,10-TETRAOXASPIRO(5,5) UNDECANE TO CONTACT EACH OTHER UNDER CONDITIONS CONDUCIVE TO CONDENSATION. SUCH A POLYIMIDE IS BLENDED WITH A CONVERTIBLE POLYESTER AND A PHENOLIC SOLVENT TO PRODUCE A COMPOSITION WHICH IS SUITABLE FOR USE AS AN ELECTRICALLY INSULATING VARNISH FORMING A COATING FILM OF HIGHLY DESIRABLE PROPERTIES.

April 10, 1973 MUNEHIKQ SUZUK] ET AL 3,726,833

POLYIMIDES OF BENZOPHENONE TETRACARBOXYLIC ACID COMPOUND AND 3,9BIS(5-AMINOPROPYL)2 4, 8, 10TETRAOXASPIRO( 5 5) UNDECANE AND METHOD OF PREPARATION Original Filed May 21, 1969 :I Q v mums-32mg;

00m 02. 00m 00m 000. O0: OON O02 00- OOQ 009 O0: O09 O09 OOON OOQN 08m OnNMOOwm OOO United States Patent O 3,726,833 POLYIMIDES OF BENZOPHENONE TETRACAR- BOXYLIC ACID COMPOUND AND 3,9-BIS(3- ABHNOPROPYL) 2,4,8,l TETRAOXASPHZO (If aig-UNDECANE AND METHOD OF PREPARA- N Munehiko Suzuki, Yokosuka, Etsuo Hosokawa, Yokollama, and Misao Waki, Kawasaki, Japan, assignors to Showa Deusen Denran Kabushiki Kaisha, Kanagawaken, Japan Original application May 21, 1969, Ser. No. 826,491. Divided and this application Jan. 6, 1971, Ser. No. 104,489

Int. Cl. C08g 20/32 US. Cl. 260-65 3 Claims ABSTRACT OF THE DISCLOSURE A polyamide having a recurring structural unit of the formula can be produced by causing a 3,3,4,4-benzophencnetetracarboxylic acid compound and 3,9-bis-(3-aminopropyl)-2,4,-8,10-tetraoxaspiro(5,5)undecane tocontact each other under conditions conducive to condensation. Such a polyimide is blended with a convertible polyester and a phenolic solvent to produce a composition which is suitable for use as an electrically insulating varnish forming a coating film of highly desirable properties.

This application is a divisional application of co-pending application Ser. No. 826,491 filed May 21, 1969 and now US. Patent 3,634,304.

BACKGROUND OF THE INVENTION This invention relates generally to polyimides and compositions thereof and more particularly to new polyimides derived from 3,3,4,4-benzophenonetetracarboxylic acid and 3,9-bis(3-aminopropyl)-2,4, 8,l0-tetraoxaspiro(5,5)undecane and new compositions produced by dissolving these imides and convertible polyesters in phenolic solvents.

Furthermore, this invention in still another aspect thereof relates to improvements in electrically insulating varnishes and more specifically to new insulating varnishes of polyimide resins which have high stability and are capable of forming coating films having, simultaneously, excellent heat resistance and mechanical properties, and for which, moreover, inexpensive phenolic solvents can be used as varnish solvents.

In recent years continual eiiorts have been directed toward miniaturization of size, elevation of performance, and expansion of fields of applications of electrical machines and equipment, such as motors and generators, in

which coils are used. In accordance with this trend, there has grown a demand for development of so-called magnet Wires which can be used even at high temperatures.

As an insulating varnish for a heat-resistant enameled wire of this character, so-called polyimide resin insulating varnishes, each prepared by causing an aromatic tetracarboxylic acid dianhydride and an aromatic diamine to react in an organic polar solvent at a temperature below 50 degrees C. and obtaining a polyamide acid solution, have been known.

When such a conventional polyimide resin insulating varnish is applied onto an electroconductive material such as copper wire or an aluminum wire and baked, the polyamide acid undergoes dehydrocyclization and converts into a polyimide, thereby forming an insulating varnish film of high heat resistance.

While these known polyimide resin insulating varnishes have excellent properties once they are applied and baked, they are accompanied by certain diificulties. One difficulty is the high price of the varnish due to the solubility of the polyamide acid constituting the predominant constituent thereof in only expensive organic polar solvents. Another difiiculty is the extreme instability of the polyamide acid, which readily converts into an insoluble polyimide even at room temperature, giving rise to gelation of the varnish, whereby the varnish must be stored under refrigeration. Such difliculties are industrially disadvantageous features of these known insulating varnishes.

Accordingly, the reaction for producing such a polyamide acid has heretofore been carried outat low temperatures, at which the tetracarboxylic acid does not react with the diamine unless it is in the state of a dianhydride. For this reason, it is disadvantageously necessary to heat the tetracarboxylic acid dianhydride for a long time prior to carrying out the reaction thereby to drive off thoroughly the adsorbed water.

Inventions relating to polyimides obtained by heating and causing reaction of an aromatic tetracarboxylic acid and an aliphatic diamine are disclosed in US. Pat. 2,710,- 853 (patented June 14, 1955, Polyimides of Pyromellitic "Acid) and US. Pat. 2,731,447 (patented Jan. 17, 1956,

Novel Polyimides). The polyimides of these inventions, however, have very low solubility of organic solvents and cannot be used in practice as constituents of insulating varnishes.

Furthermore, baked varnish films obtained through the use of the above described known insulating varnishes of polyimide resins are deficient in various mechanical properties beginning with wear resistance, whereby when a magnet wire on which any of these varnishes has been applied and baked is wound into a coil by an automatic winding operation, there is a tendency for the varnish film to be damaged.

Heretofore, the technique of improving the various properties of an insulating varnish film by admixing one or more resins amply possessing the desired properties with the insulating paint has been known. However, known polyimide resin insulating varnishes as described above have no mutual solubility whatsoever with resins such as convertible polyesters which have excellent mechanical properties. Consequently, it has not been possible to resort to the above mentioned technique of improving varnish film properties in the case of the above described known polyimide resin varnishes.

In view of the above described state of the art, we have studied the above described problems. As a result we have discovered that polyimides derived from a certain tetracarboxylic acid and a certain diamine are readily soluble in inexpensive phenolic solvents and that, moreover, convertible polyesters can be dissolved in large quantities in the resulting polyimide solutions, whereby it is possible to produce an insulating varnish capable of forming an insulating varnish film which possesses, simultaneously, heat resistance and excellent mechanical properties.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide polyimides which are soluble in phenolic solvents.

Another object of the invention is to provide compositions each comprising a polyimide of the above stated character, a convertible polyester, and a phenolic solvent.

A further object of the invention is to provide insulating varnishes of new polyimide resins each having, as prin cipal constituents thereof, a polyimide and at least one polyester.

According to the present invention in one aspect thereof, briefly summarized, there is provided polyimides each having a recurring structural unit of the formula O-GH: CH2O\ --CH:CH:CHz-CH G CHCHzCHzCHa-N O-CH: CHz-O According to the present invention there is further provided a method for producing polyimides of the above stated character.

According to the present invention, in still another aspect thereof, there are provided compositions each comprising as a blend a polyimide of the above stated character, a convertible polyester, and a phenolic solvent. One application of these compositions is their use as electrically insulating varnishes.

The nature, details, and utility of the invention will be more clearly apparent from the following detailed description beginning with general considerations and concluding with specific examples of practice constituting preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWING In the accompanying drawing, the single figure is an infrared analysis chart of a polyimide suitable for use according to the invention.

DETAILED DESCRIPTION A polyimide according to the invention can be produced, in general, by: blending 3,3,4,4-benzophenonetetracarboxylic acid (hereinafter designated by the ab-- breviation BTA), a functional derivative thereof as, for example, a dianhydride of BTA (hereinafter designated by the abbreviation BTDA), or a lower alkylester of BTA, and approximately the same mol quantity of 3,9-bis(3- aminopropyl) 2,4,8,1O tetraoxaspiro(5,5)undecane of the formula H N-CI-I 'CH (hereinafter designated by the abbreviation BTU) together with a phenolic solvent in proportions such that the sum of the quantities of the two first-named components (i.e., -BTA or a functional derivative thereof or a lower alkylester of BTA and BTU) is from 5 to 60 percent by weight of the total sum of the entire batch thus blended; placing the batch in a suitable reaction vessel; and agitating the batch for a period of from 1 to 6 hours at a temperature above degrees C.

The mol ratio of the above mentioned two components need not strictly be equal. Rather, a blend in which the quantity of one reaction component is somewhat in excess of that of the other component is even more preferable.

However, if the difference between these quantities is excessive, the degree of polymerisation of the polyimide produced will be restricted with the result that the characteristics of the baked coating film of the insulating varnish will be impaired. Accordingly, this difference is preferably held Within 10 mol percent of the smaller quantity.

We have found further that when the quantity of the reaction solvent is excessively large relative to that of the reactants, the reaction does not progress thoroughly. On the other hand, when the quantity of the solvent is deficiently small, the concentration of the resulting polyimide solution becomes excessively high, which result is undesirable since it becomes difficult for the convertible polyester added in the succeeding process step to dissolve.

In this reaction, furthermore, the reaction temperature has a great influence on the progress of the reaction.

3 no C O More specifically, this polyimide-forming reaction may be considered to take place in accordance, for example, with the following dehydration reactions of two steps (dehydration and dealcoholisation reactions in the case Wherein a lower alkylester is used).

where X represents and Y represents We have found that when the reaction temperature is below 80 degrees C., the reaction indicated by Formula II progresses with difficulty, and the reaction solution becomes one in which the proportion of the polyamide acid is high. As a result, the mutual solubility with the convertible polyester added in the succeeding process step is lowered, and, in addition, blisters or pin holes tend to be formed in the coating film since dehydration occurs at the time of baking of the varnish film.

Furthermore, since the reaction of Formula I is also a dehydration reaction, the reaction system will reach an equilibrium state if it is difiicult for the separated Water to escape out of the system, whereby it will be difiicult to obtain a polyimide of high molecular weight.

Accordingly, it is preferable that the above described reaction be carried out at a temperature above 100 degrees C. and below the boiling point (from 200 to 220 degrees C.) of the solvent.

When the reaction is carried out under reduced pressure, a reaction temperature which is lower than that in the case of reaction at atmospheric pressure can be used. Furthermore, the reaction temperature can be temporarily lowered below 80 degrees C. during the reaction process without any adverse effect.

A polyimide produced in the above described manner is characterised by an infrared analysis chart as shown in the accompanying illustration and readily dissolves in a wide range of proportions in phenolic solvents. Examples of such phenolic solvents are phenol, xylenol, and cresol. Poor solvents such as naphtha, toluence, and xylene can also be used when the additive quantities thereof are below 60 percent.

The polyimide thus produced, in general, is substantially totally soluble (at 30 degrees C.) in the phenolic solvent and, in general, has a relative viscosity (0.5 g. polymer/IOO mlpm-cresol, 30 degrees C.) higher than 0.1. The m-cresol used for this relative viscosity and the examples of practice to follow was of the following composition.

Percent Phenol 3.2 o-Cresol 3 .3 p-Cresol 39.6 m-Creso1 5 3.9

A polyimide produced in the above described manner, in the form of a solution, can be used directly, as it is, as an insulating varnish capable of forming a heat-resistant coating film. However, the baked coating film resulting from this solution is not always completely satisfactory in mechanical properties.

In accordance with the present invention, for the purpose of improving these mechanical properties of the baked coating film, a known convertible polyester or a solution of a convertible polyester in an organic solvent, that is, a polyester insulating varnish, is further added to the above mentioned polyimide solution, and the resulting combination is uniformly mixed. At this time, a'diisocyanate or isocyanate generator may, of course, be added simultaneously together with the convertible polyester.

The convertible polyester added in this manner is mutually dissolved with the polyimide solution in almost any ratio to impart a substantial improvement in the properties of the product over those of the polyester insulating varnish by itself or those of the polyimide solution by itself even when, with respect to parts by weight of the resin content within the polyimide solution, a small quantity of the order of 10 parts by weight of the convertible polyester is added or, conversely, when a large quantity thereof of the order of 1,000 parts by weight is added.

The quantity of the diisocyanate or isocyanate generator to be added is preferably less than 50 parts by weight with respect to 100 parts by Weight of the resin content of the insulating varnish (polyimide solution plus convertible polyester).

The term convertible polyester is herein used to desigate a so-called primary condensate which is soluble in a phenolic solvent and has heretofore been used as the polyester component of polyester insulating varnishes, and which is a polyester capable of undergoing further condensation reaction under the action of heat and (or) a catalyst to be converted into a cross-linked structure. This convertible polyester can be prepared, in general, by heating and causing reaction of a mixture of suitable proportions of a dicarboxylic acid, a divalent alcohol, and an alcohol or an acid of a valency of three of thereabove as a cross-linking component in a phenolic solvent.

Therefore, this polyester is one which has a hydroxyl group and a carboxyl group which are capable of reacting further to form a three-dimensional network and are in a free state or a state wherein the reactivity thereof is blocked because of etherification or esterification with a lower alcohol.

Examples of dicarboxylic acids suitable for use as one of the above enumerated materials are terephthalic acid, isophthalic acid, adipic acid, pimelic acid, and suberic acid. In addition, lower alkylesters of these acids can be similarly used. Examples of suitable divalent alcohols are ethylene glycol, diethylene glycol, trimethylene glycol, tetramethylene glycol, butanediol, and 2.2-dimethyl-l.3- propanediol. Examples of trivalent or higher valency alcohols are ethylene glycol, diethylene glycol, trimethylene glycol, tetramethylene glycol, butanediol, and 2.2- dimethyl-1.3-propanediol. Examples of trivalent or higher valency alcohols or acids are glycerine, trimethylolpropane, pentaerythritol, tris (hydroxymethyl) isocyanurate, diglycerol, cyanuric acid, isocyanuric acid, and trimellitic acid. Lower alkylesters of these acids can be similarly used.

Among the various possible combinations of the above described starting materials, one suitable combination we have found comprises approximately equal quantities of terephthalic acid, ethylene glycol, and glycerine. We have found further that substitution of a portion of this combination with another acid or alcohol also produces desirable results.

Furthermore, the convertible polyester in the present invention includes those of modified as, for example, a polyester prepared by further adding a small quantity of a reactant such as an unsaturated fatty acid to the starting materials, as set forth in the specification of British Pat. 1,088,323, or a convertible polyester already produced to which a modifier such as a diisocyanate has been added.

A convertible polyester suitable for use according to the invention can be produced as illustrated by the following specific example of procedure. The above described starting materials, together with a phenolic solvent in a quantity of the order of 20 percent by weight of the total weight thereof, are placed in a suitable reaction vessel and heated from room temperature to a maximum temperature of from 230 to 250 degrees C. at a temperature rise rate corresponding to a time of from 6 to 9 hours for this temperature rise thereby to product the convertible polyester as a reaction product.

While this reaction is completed after the reactants have been heated to a final temperature of approximately 230 to 250 degrees 0., the molecular weight of the reaction product thus produced can be further increased by maintaining the product at the final temperature for a period of from 1 to 4 hours. It is necessary however, to cause the reaction to stop prior the start Accordingly, a mixture of 1 mol of BTA and 1 mol of BTDA will be considered to be identical to BTA monoanhydride. Furthermore, a mixture of 1 mol of BTA and 1 mol of a tetra-alkylester of BTA, a mixture of 1 mol of a mono-alkylester of BTA and 1 mol of a 5 of gelation of the resin formed in carrying out this protri-alkylester of BTA, and a mixture of 1 mol of BTA, cedure of holding the reaction product at the final tem- 1 mol of a mono-alkylester of BTA, 1 mol of a di-alkylperature. For this purpose, a phenolic solvent is poured ester of BTA, 1 mol of a tri-alkylester of ETA, and 1 onto the heated convertible polyester. The convertible mol of a tetra-alkylester of BTA will each be considered polyester obtained in this manner is then filtered to reto be identical to a di-alkylester of BTA. move foreign matter and then used. The acid values and other characteristics of BTA and Convertible polyesters of the above described characanhydrides and lower alkylesters of BTA are set forth in ter suitable for use according to the invention include Table 1. polyester resin solutions, i.e., polyester insulating var- TABLE I nishes sold on the market under trademark or product names such as: Alkanex (General Electric Company, fj} Mo1ec U.S.A.), Terebec (Farbenfabriken Bayer AG, Germany), c d Abbrevia- Acid esteriular each of which is a polyester insulating varnish synthesised ompmm tum Value ficatmn weight from dimethyl terephthalate, ethylene glycol, and glyc- 3.3,4 4-b 9% tw A 6 5- 858 erine; Isonel (Schenectady Varnish Company, U.S.A.), 2 3 %i BTDA 322 which is a polyester insulating varnish synthesised from ggi r A 33 terephthalic acid, ethylene glycol, glycerine, and tris (hy- BTA ijf BTA M 290:2 i 386 droxyethyl) isocyanulate; and Desmophen (Farbenfabri- BTA trimethylester 0- 400 BTA tetramethylester... $4 414 ken Bayelr A?21 Germany). t Egg gl ggfimg 8 2 9y 23g Examp es 0 iisocyanate or isocyanate generators sui 25 'P Y 85 BTAE able for adding to the insulating varnishes of the inven- Hi g ig g 'fifgg 123'3 Z 25 tion are diphenylmethane-4.4'-diisocyanate, diphenylether-4-4'-diiS0cYaI1ate, diphenylsulphide diisocya- In order to indicate still more fully the nature and Hate, diphenylsulphone di i socyanate, dlphellylbenutility of the invention, the following specific examples zene-4.4'-diisocyanate, tolyl n d11S0C yaI1ate, e e y of practice constituting preferred embodiments thereof one dii y octamelhylene dilsocyarlate, P11611011c and results are set forth, it being understood that these stabilisers of these diisocyanates, and isocyanates sold on examples are presented as ill t ti nl d th t th the market under product names such as Desmodur AP r t int nd d to 1mm th scope of th inv ti Stable, Desmodur CT Stable, Desmodur T, Desmodur N, EXAMPLE 1 Desmodur TH, and Desmodur R (Farbenfabriken Bayer AG, Germany). 322.2 grammes (g.) of BTDA, 274.4 g. of BTU, and The preparation of the polyimides for use in accord- 9 xylem] were Placed m a three'neck flasl? ance with the invention will be disclosed hereinafter with vlded wlth thermfmletel" condenser and agitator respect to a specific example of method. As mentioned 40 g tg wlthltn flask fi g gamed hereinbefore, while the BTA itself can be used in its acid fs i z g f z S 33 '3 a e g was e at form, an anhydride or a lower alkylester thereof can be or w e e ag Ion similarly used tlnued, the reactlon bemg stopped when generation of L I water vapour completely stopped, whereupon a polyimide Accordmgly not only A but i teha was obtained. The quantity of water distilled during the alkylesters of BTA can be isolated with pur1t1es above reaction was 35 ml. 99 percent, but it 1s difiicult, strictly speaking, to isolate the monoanhydride and mono-, di-, and tri-alkylesters of EXAMPLES 2 THROUGH 14 BTA. Therefore, these compounds will be identified by In a manner similar to that set forth in Example 1, their acid values or degrees of esterification in the followpolyimides were prepared under the blending and reaction mg examples. conditions set forth in Table 2.

TABLE 2 Acid com- Reaction ponent/ conditions Pressure of Ex. Acidcom- Acid BTU (mol Reaction solvent (Quantity temp. C.)X reaction ponent value rati blended) time (hr.) system 696.9 11x1 160 5o 2 620 1/1 1 nitiieshgiiw i202 iil i iiir ii i 332 1/1 Ph 1(40%) 130 2 100 mm. Hg. 462 1. 1 170X2 20mm. Hg. 287 200X1 Atmospheric. 135 130 2 20mm. Hg. 0 180X2 20mm. Hg. 424 180X2 20mm.Hg. 270 1s0 2 20mm.Hg. 120 1 1 1s0 2 20mm. Hg. 0 d0 180X2 20mm. Hg. 620 1/ Xylenol/naphtha 4/6 50X1.5 150 mm. Hg.- 696.9 1.1/1 Xy1enol/xylene=9l1(60%) X1;150X2 Atmospheric; 696.9 1/1.l Xyienol/toluene=9.5/9.5 (60%)--.. 80 6 mm. Hg:

Each of the polyimide solutions obtained in the above described manner was dropped into methanol to cause precipitation, and the precipitate was washed several times with methanol and then dried under a vacuum at room temperature, whereupon a polyimide in the form of lightl EXAMPLE 15 A solution of Desmophen F 950 in xylenol was added to the polyimide solution prepared by the procedure of Example 1 in a quantity such that 100 parts of the resin 110 d bt d content of the Desmophen solution was added with reg g g g F ug t m thod) a 6 Hi d out spect to 100 parts of the resin content of the polyimide with 3: of th e f a n a solution. The resulting mixture was uniformly mixed by ha i th i; 3 fi agitation, and a suitable quantity of xylenol was therel e F; L cm after added further thereto thereby to produce a varnishg a c o S ul 6 like solution having a solid content of 40 percent. ThlS ifi 2 q 5 3123. g l 3 re resin solution was filtered to remove foreign matter. i i 2 2? y 15 Yi S ct on i 16 The filtered resin solution was then applied as a coating S i ma 6 fl 2 ii z 3 z directly on a annealed copper wire of 1.0-mm. diameter w i 5 was ven e a e P s ere and baked in the conventional manner at a temperature Po yum 15 of from 350 to 450 degrees C. thereby to produce a Measglred Calculated enameled wire.

V llB va 116 Element (percent (percent) EXAMPLES 16 THROUGH 33 64-43 64-28 By the procedure set forth in Example 15 other resin 4.70 5. 04 solutions respectively of the blends and particulars as shown in Table 3 were produced.

TABLE 3 d (B) (C) (D) Non-volatile; (F) Polyimi 0 con en I solution (A)/(B)/(( 3) (percent) viscoslty (Example Convertible polyester or (wt. ratio (200 C. 1.5 (poise) Example No. N o.) polyester varnish Diisocyanate or isoeyanate generator of solids) hr.) at C 1 DesmophenF9' 100/100/0 40.0 38.2 2 Alkanex 9'04 100/10/0 41.2 41.5 3 do 100/1. 000/0 40. 2 37.4 4 Isonel200 100/20/0 30.9 40.4 5 TerebeeF.. 100/900/0 42.0 39.15 6 Desmophen 800 Diphenylmethane-4.4-diisoeyanate 100/400/10 38.7 36.3 7 Desmophen 900. Diphenylether-4.4-diisoeyanate 100/400/1 41.8 38.5 8 Desmophen 1000. Diphenylsulphide-4.4-diisocyanate 100/400/250 39.5 37.2 6 Desmophen 1100 Diphenylsulphone-4.4-diisoeyanate 100/400/20 40.7 40.7 10 Desmophen1200- Dlphenylbenzene-4.4-diisoeyanate 100/100/10 40.5 38.0 11 Desmophen 2000. Tolylene diisocyanate 100/100/10 40.1 38.1 12 Desmophen 2100- Hexamethylene diisoeyanat 100/200/5 39.3 36.9 13 Desmophen 2200- Octamethylene diisooyanate-.. 100/200/3 39. 6 36. 0 14. Desrnophen VL 100-. Desmodur AP stable 100/300/5 4.0.9 39.6 15 Alkanex 9504 Desmodur CT stable 100/400/10 38.8 34. 5 16 Isonel 200 DesmodurR 100/40 6 41.4 37.9 17 Terebec F 100/50/0 40.7 41.0 18 Isonel200 100/900/0 40.4 39.2 19 Alkanex950 100/100/0 38.6 35.7

The principal properties of this polyimide were as follows.

N-methyl-2- The following Examples 15 through 33, inclusive, relate to insulating varnishes of polyimide resins according to the invention. While convertible polyester solutions sold on the market are used in these examples for reasons of convenience, it should be understood, of course, that the present invention is not limited to the use of only such polyesters but can be practiced with any polyester falling within the scope of convertible polyesters defined hereinbefore.

The resin solutions thus produced were applied as coating on annealed copper wire of 1.0-mm. diameter similarly as in Example 15 and baked, whereby respective enameled wires were obtained. The insulating film thicknesses of these enameled wires were all within the range of from 0.046 to 0.047 mm. The characteristics of each of these enameled Wires are indicated in Table 4.

For the purpose of comparison, corresponding characteristics of enameled wires, designated as Reference Examples Ref. 1 through Ref. 4, inclusive, are also shown in Table 4, these reference enameled wires being produced by applying as coating polyester insulating varnish and polyimide resin varnish independently on annealed copper wire of 1.0-mm. diameter and baking the applied varnish under the same conditions as in the other examples.

Alkanex 9504 was used in Reference Example Ref. 1, Terebec F in Ref. 2, and Isonel 200 in Ref. 3, and in Ref. 4 use was made of a polyimide resin insulating varnish (a solution in an organic polar solvent of a polyamide acid synthesised from pyromellitic acid dianhydride and 4.4'-diaminodiphenylether) sold on the market under the product name of Pire-ML (E. I. du Pont de Nemours & Co., Inc., U.S.A.).

It was found that a varnish in which a polyimide solution for use in the invention was used by itself produced a coating film which lacked bakeability and, on a annealed copper wire, developed cracks when the wire Was wound around a wire of its own diameter.

2. A method for preparing a polyimide consisting esup to 60% by volume of the mxture of a hydrocarbon s01- sentially of recurring structural units each representable by vent. the formula:

ll E

which method comprises heating a mixture of (1) a tetra- References Cited carboxylic acid compound selected from the group con- 15 UNITED STATES PATENTS s1st1ng of 3,3 ,4,4 -benzophenone-tetracarboxylrc acid, an- 2 hydrides thereof and lower alkyl esters thereof and 2 a 3 53%; Z132; 22 -b' l -2,4,8,10-

damme of (3 ammopmpy) 3,349,061 10/1967 Pruckmayr 26047 tetraoxaspiro (5,5)-undecane in substantially equimolar ro ortions in a henolic solvent at a tern erature at vhi zh Water boils f or a time sufiicient for corngletion of a 20 WILLIAM SHORT Pnmary Exammer ring-closing reaction of said tetracarboxylic compound with L. L. LEE, Assistant Examiner said diamine compound into said polyimide.

3. A method for producing a polyimide as claimed in CL claim 2 in which said phenolic solvent in admixture with 25 l17128.4; 26033.4 P, 857 R 

